Hydrophobic nucleation in particle agglomeration

Agglomeration of poorly wetting powders is a common problem in pharmaceutical, minerals and fertilizer granulation. There are very few clear studies of poorly wetting systems, although many papers contain a statement to the effect that good wetting is a prerequisite for good nucleation in particle agglomeration. Although this is usually true, robust granulations with a poorly wetting powder is possible and is reported in this study. A mixture containing about 70 weight % (90% by volume) of a micronized poorly wetting powder was granulated in a high shear mixer using water without surfactant, and the microstructure of resultant agglomerates (granules) was studied using optical and electron microscopy as well as X-ray computed tomography (XRCT). The study revealed that granules are typically spherical or elliptical in shape and range in size from 200 to 500 microns. They are strong and consist of a consolidated powder shell and an empty core. Based on the microstructure, a nucleation mechanism for such a hydrophobic system is proposed. Implications for controlling granule growth and granule properties are discussed.